1. Field of the Invention
The present invention relates to a printed interconnection board and a method of manufacturing the same, and in particular, to a printed interconnection board having a core including carbon fiber reinforced plastic and a method of manufacturing the same.
2. Description of the Background Art
In recent years, it has become desirable for printed interconnection boards to include a substrate with an excellent heat dissipation property, as electronic components are mounted in higher density. A metal core substrate has been known as a printed interconnection board excellent in heat dissipation property, and has already been put to practical use. The metal core substrate uses a metal having high thermal conductivity, such as aluminum (Al), copper (Cu), or the like, as a core material, and thereby can dissipate heat from a heat-generating component throughout the substrate and suppress an increase in the temperature of the heat-generating component. Above all, aluminum, which has a low specific gravity, is generally used as a core material.
However, aluminum has a high coefficient of thermal expansion of approximately 24 ppm/° C., whereas a ceramic component has a low coefficient of thermal expansion of approximately 7 ppm/° C. Therefore, there arises a problem that when a heat cycle test is conducted, a crack occurs at a solder joint portion due to a difference in the coefficients of thermal expansion of aluminum and the ceramic component, failing to achieve mounting reliability.
As a core material capable of solving the above problem, carbon fiber reinforced plastic (hereinafter referred to as CFRP) has been known (for example, see Japanese Patent Laying-Open No. 11-040902). CFRP is a composite material including carbon fiber and resin, and has properties such as low thermal expansivity (±2 ppm/° C.), high thermal conductivity (140 to 800 W/m·K), and low specific gravity (1.6 g/cm3). If a core substrate can be fabricated using this CFRP, a substrate with high thermal conductivity and more excellent in mounting reliability than aluminum can be obtained.
Since a CFRP core substrate possesses electrical conductivity as with other metal cores, it is necessary to insulate it from a penetrating through hole for connecting interconnections provided above and below the core.
However, there has been a problem that, in a substrate using CFRP prepared by laminating prepregs including unidirectional carbon fiber at angles of 0°, 90°, 90°, and 0° as a core material, exfoliation occurs in a CFRP layer on a side surface of the substrate in a heat cycle test. This is attributed to the fact that stress caused by a difference between a coefficient of thermal expansion of the CFRP and coefficients of thermal expansion of a substrate material and copper is stronger than adhesion strength between the carbon fiber and resin of the CFRP.
Further, since the CFRP layer is exposed on the side surface of the substrate, there has been a risk that electrically conductive carbon powders may fall off, adhere to between interconnections of the substrate or an insulating portion of a device, and cause a short circuit between the interconnections. In particular, when exfoliation occurs in the CFRP layer at an end portion of the substrate in a heat cycle test, falling-off of carbon powders has exerted a significant effect at an unacceptable level.